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Circular railgun?

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hen918

Thu Jan 22 2015, 06:14PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

As DerAlbi says a railgun with aluminium (on its) rails would be a one shot device. Tungsten is a good material for high temperatures / wear but you would still get deposits off of the steel projectile.

Shrad, If you use conductive powder / lubricant you risk shorting the rails out and it arcing over.

EDIT: oh, and the Lorenz forces are going to try and force the rails apart, so if they aren't completely rigid energy is going to be wasted that way.
EDIT2: The projectile will have to be moving when it hits the rails. Theoretically it is possible to do it stationary, however it is very difficult to do it that way practically.

Ash Small

Thu Jan 22 2015, 06:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Two steel rings (or other suitable metal) held close to each other and a mechanism for moving the rings apart for projectile release.

Only drawback is timing the release, so you can control direction, but if the mechanism is surrounded by a spiral of one turn (if that makes sense), you have directional control as well.

BigBad

Fri Jan 23 2015, 12:02AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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The tricky bit with a spiral like that is that if it releases at the wrong moment the ballbearing will go slamming into the start of the spiral.

Ash Small

Fri Jan 23 2015, 02:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

The tricky bit with a spiral like that is that if it releases at the wrong moment the ballbearing will go slamming into the start of the spiral.

I realise this. Some form of 'timing mechanism' is required. The 'separation mechanism' needs to be triggered at a point that aviods this. The exact detail of the timing mechanism will depend upon the properties of the rest of the system.

klugesmith

Fri Jan 23 2015, 04:56PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Ash Small wrote ...
Only drawback is timing the release, so you can control direction, but if the mechanism is surrounded by a spiral of one turn (if that makes sense), you have directional control as well.

If the track is a finite helix instead of a circle, there's a place for injecting the projectile and a place for letting it fly away. As with any very long railgun, you need to worry about voltage drop in the rails.

Correct me if I'm wrong, but the electromagnetic propulsion depends on the rails carrying current _behind_ the projectile and no current ahead of the projectile. A circular rail (for benchtop "spinner" experiments) needs at least one electrical gap next to a power connection. That will define the direction of rotation.

That's in contrast with the classic "ball bearing motor" trick. I can attest that those run in whichever direction you choose to start them. Fun thing to do with a low-voltage rewound MOT, when you get tired of abusing nails and small wires.

Shrad

Fri Jan 23 2015, 05:02PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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voltage drop in the rails could be addressed with multiple parallel connections to several parts of the path, no?

for the release mechanism, a sensor at an interval x+k of the release point and another one at an interval x of the first sensor, a simple timing circuit repeating the timing between sensor 1 and sensor 2 triggered by sensor 2, and a modifier k for setting delay in mechanical device used for releasing the projectile

klugesmith

Fri Jan 23 2015, 05:16PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Shrad, I think you're missing the point of my second paragraph (in prev. post), or I am wrong.

Let's start with a conductive bridge (or sliding wire, in the EM textbooks) carrying current between two rails, and a current source connected to one end of the rails. There's an EM force tending to push the bridge away from the current source, increasing the rectangular loop's area and inductance.

Suppose the rails are powered from both ends. What defines the partition of bridge current between the two sources? Relative resistance of the rail sections? If the currents match then there's no propulsive force. I think it makes no difference if the bridge is moving to begin with.

Ash Small

Fri Jan 23 2015, 06:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Place a gap in the rings, so the projectile repeatedly starts at the beginning.

If only one side of the rings 'opens up', this gives us a lot more directional control, but some timing mechanism would still probably be required. If the projectile breaks a light beam it tells where it is, and the time taken to circle, which gives us it's speed.

EDIT: Pneumatic rams may be useful here.

BigBad

Fri Jan 23 2015, 06:47PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Shrad wrote ...

voltage drop in the rails could be addressed with multiple parallel connections to several parts of the path, no?

Yes, but you'd need multiple breaks in the rails as well, as others have noted.

But it's less of an issue, because the current and resistance losses are smaller anyway.

Shrad

Fri Jan 23 2015, 09:00PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

that would be a nice experiment... a steel ball, two copper tubes and arewound MOT ^^

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